GENUS AMPHISTEGINA : — OEGANIZATION. 
33 
being still continued over it, but the cavity of the chambers being limited to the peri- 
pheral portion of the whorl. Thus the form of the shell is so completely altered, that 
it would not be recognized as the same if the successive stages of this alteration could 
not be traced, especially as the characteristic radiating lines are no longer to be seen on 
the prominent centre. Its diameter undergoes an extraordinary increase ; the specimen 
from which fig. 17 was taken measuring no less than '30 inch by *25 inch. 
168. The organization of the interior of the shell, as displayed by sections passing 
either through the median plane or parallel to it (Plate VI. fig. 6), and by sections 
taken perpendicularly to the median plane, or vertically (Plate VI. fig. 6), is closely con- 
formable to that of O^ercuUna ; the chief difference arising out of the extension of the 
alar prolongations of the chambers of each successive whorl, and of their dividing septa, 
over the entire surface of the penultimate whorl, except at the centre of the spire, which 
is conunonly occupied by a solid pillar of non-tubular substance, having the general 
shape of a double cone, whose apex is in the primordial cell, and whose base is on the 
most prominent part of each surface. As it is very seldom that the successive whorls lie 
precisely on the same level, a section which traverses the outer convolutions in the 
median plane will pass above or below the plane of the inner whorls ; and it will thus, 
in traversing the surfaces of these, bring into view the centripetal prolongations of the 
septa intervening between the alar prolongations of the chambers of the whorls which 
invest them, as is shown in the central portions of fig. 6. These centripetal pro- 
longations are composed of non-tubular shell-substance, and the portion of the spiral 
lamina above them is also non-tubular; and thus it is that their position is marked 
on the exterior by radiating bands, which never rise in A. Oumingii into tubercles or 
ridges, though they sometimes become thus elevated in A. gibbosa. These septal 
bands are, generally speaking, those of the last-formed whorl alone, those of all the pre- 
ceding whorls being concealed by the investment they have received ; but in unsymme- 
trical specimens (as often happens in A. gibbosa, and occasionally in A. Cumingii), the 
alar prolongations do not extend to the centre of the flatter side ; so that the septal 
bands which radiate from the centre are those of the penultimate whorl, and are inter- 
rupted as they approach the margin by a new series belonging to the last-formed con- 
volution. The reason of the obliteration of the septal bands on the prominent portion 
of the shell in advanced specimens of A. Cumingii is at once explained when we look at 
fig. 5 ; for as the last whorl extends itself peripherally, the alar prolongations ot its 
chambers and therefore of its septa are withdrawn from the central region, and thus no 
new septal bands will be formed ; whilst the continuation of the thick spiral lamina over 
the whole surface of the penultimate whorl prevents its septal bands from displaying 
themselves. In this respect there is a marked contrast between A. Cumingii and A. gib- 
bosa ; for on the surface of the oldest specimens of the latter we see not only the nume- 
rous septal bands of the outer whorl, but indications of those of the penultimate whorl, 
which show themselves as imperfect or broken lines between the complete radiating 
bands. This is probably due to that elevation of the septal bands of which mention has 
MDCCCLIX. 
F 
